Project Summary Cochlear implant (CI) electrode arrays are made of platinum (Pt) wires and contacts encased in polydimethylsiloxane (PDMS, silicone) housing. These materials provide mechanical stability and flexibility critical to the long-term function of the device. However, they also induce a foreign body response and fibrosis that have detrimental effects. For example, the fibrotic capsule that eventually encases all CI electrode arrays leads to increased impedances and signal broadening which decreases the effectiveness of the device. Further, intracochlear fibrosis is implicated in the loss of acoustic hearing that can occur months to years after implantation. As candidacy for CI is rapidly expanding, including many patients with significant residual hearing, there is an urgent need to understand the fundamental processes that lead to intracochlear fibrosis. Macrophages are recognized as key, central regulators of the foreign body response to biomaterials in other tissues and our preliminary data demonstrate vigorous macrophage recruitment following implantation of CIs. We hypothesize CI biomaterials activate macrophages leading to the recruitment of fibroblasts and fibrosis/encapsulation of the biomaterials and that electrical stimulation modulates this macrophage response dependent on the stimulus level and timing of onset. In Aim 1, in vitro culture models are used to explore the differential effect of PDMS and Pt on macrophage recruitment, activation, and regulation of cochlear fibroblast proliferative and synthetic functions. Aim 1 also investigates the temporal and spatial activation and recruitment of macrophages following cochlear implantation using a reporter mouse model. Aim 2 examines the role of macrophages in fibrosis/neo-ossification following cochlear implantation. First, we test the requirement of macrophages for intracochlear fibrosis following CI using a mouse line that allows conditional and selective depletion of macrophages. Next, implanted mice are treated with a specific CSF1R inhibitor to deplete macrophages as a preclinical translational model. Finally, a CX3CR1 null mouse is used to determine the effect of fractalkine signaling on post-CI fibrosis. Aim 3 determines the effects of varying levels of electrical stimulation and effects of timing of electrical stimulation onset on macrophage recruitment and intracochlear fibrosis. The proposed work provides a rigorous investigation of the effects that specific biomaterials, insertion trauma, and electrical stimulation exert on macrophage responses and the regulation of the fibrosis in the cochlea. The long-term impact of the work is to identify specific, effective, and durable strategies to limit fibrosis following CI or other injuries to the cochlea.